In recent years much progress has been made in our understanding of the basic engineering science of articular cartilage lubrication, driven significantly by the progress achieved in the prior funding periods (1995- 2008) of this grant. Though many competing hypotheses had been advanced over past decades, starting from the 1930's, a fundamental mechanism of lubrication driven by interstitial fluid pressurization of articular cartilage has now been identified and validated from theory and numerous experiments. Upon loading, the interstitial fluid of contacting articular layers pressurizes significantly, supporting most of the contact force; consequently, only a small fraction of this contact force is supported by the contacting collagen-proteoglycan matrixes, producing a negligible friction force, and thus a low friction coefficient. Though boundary lubricants present in synovial fluid help to further reduce the friction coefficient, our recent study has shown that the dominant reduction in friction is contributed by this interstitial fluid pressurization mechanism. If the interstitial fluid pressure subsides, as may occur under certain loading conditions, the friction coefficient rises dramatically, and evidence from our experiments indicates that cartilage wear increases concomitantly. Thanks to the availability of a validated theoretical framework explaining the mechanism of lubrication by interstitial fluid pressurization, it is possible to anticipate those loading conditions where interstitial fluid pressurization may subside. The specific aims of this competing continuation application are to translate these basic science findings into important, clinically relevant insights for joint hemiarthroplasties. Hemiarthroplasty is a surgical procedure that replaces one half of a diarthrodial joint with a smooth impermeable artificial bearing surface, leaving the apposing articular layer intact. Clinical experience suggests that, following hemiarthroplasty, degeneration of the native articular layer may progress at a faster rate than expected in osteoarthritis. The overall objective of this application is to a) investigate whether hemiarthroplasties fare poorly because they produce elevated friction coefficients by failing to promote sufficient cartilage interstitial fluid pressurization; and b) investigate whether hemiarthroplasties where the artificial bearing surface is a deformable porous-permeable material can produce lower sustained friction and wear than traditional impermeable bearing materials. Additional objectives aim to further deepen our basic science understanding of cartilage lubrication and wear, setting the stage for designing improved bearing surfaces for hemiarthroplasties.